Light-emitting diodes are used in a wide variety of applications. Specifically, high-power LEDs are finding widespread used in retrofit lamps; in automotive lighting modules such as daytime running lights, brake lights and indicator lights, etc. For linear lighting modules consisting of an array or series of individual LEDs positioned at a certain pitch, regions of concentrated high luminance light are observed when looking at the light source. This “spottiness” is due to the brightness of each LED and the necessary pitch between the LEDs, and is generally perceived as undesirable. Various approaches have been tried in order to obtain the appearance of a homogeneous light source when the light source in fact includes multiple LEDs. For example, a phosphor layer can reduce the inhomogeneity of the apparent light source. Alternatively, it is known to use optical scattering elements to diffuse the light originating from the LEDs. For example, scattering particles can be suspended in a translucent encapsulant layer close to the LED dies, in a further translucent layer over an encapsulant layer, or even at a remote position from the LEDs. Scattering can also be achieved by arranging a suitably treated glass cover or polymer foil about the LED module.
In another approach, the glare of the individual LEDs can be tempered by means of a functional scattering layer that is essentially transparent, but also contains elements that reflect and/or deflect the light, for example titanium dioxide particles, or trapped air bubbles. It is also possible to suspend transparent materials with a high refractive index in an encapsulant layer, for example fillers such as glass or polymer spheres. The concentration of the scattering agent—which can comprise particles, refractive glass spheres, air bubbles etc.—to obtain a desired effect will depend on various factors such as the light emitting surface area of the LED, the LED pitch, the light escape angle, the amount of lumens emitted by the LED, the optical path of the light rays, etc. For mid-power and high-power LED applications, a translucent layer is generally made of a clear silicone product such as polydimethylsiloxane. Such silicones retain their transparency over time, and their flexible rubber-like properties make them the preferred choice for flexible LED modules. The silicone is available as a two-component product, which must be prepared by thoroughly mixing both components. Scattering agents are added at the mixing stage. The prepared mixture is then poured into a mould and then cured. Using the known approaches, it is generally only possible to obtain scattering layers in which the scattering agents are more or less evenly distributed throughout the translucent material, but the LED light sources each have a local presence in the LED module. Since the scattering agent concentration must be chosen to provide the desired degree of scattering in the region optically close to an LED, the disadvantage of the known approaches is the unnecessary loss of light in any region that is not in the direct vicinity of an LED—for example regions between LEDs of an array, regions along the outer edges of an LED module, etc.
Another disadvantage of the known approaches is that a scattering agent such as titanium dioxide reduces the flexibility of the silicone encapsulant. In addition to the problems mentioned above, such an approach would therefore not be suited for a flexible LED module. Another problem associated with the filled silicone scattering layers described above is that these must be defined or configured early on in the design stage due to the requirements of the encapsulating process, essentially ruling out any late-stage adjustment of a silicone diffuser. The use of a diffuser foil is also not suited to flexible LED lighting solutions since a foil are not truly flexible in all directions.
DE 10 2013 106 689 A1 discloses a semiconductor part comprising a single LED embedded in an opening in a mould and covered by an rigid optical plate. The optical plate is profiled, which profile has a reflectivity that from above the LED decreases outward.
US 2014/160752 A1 discloses a light output device with a number of LEDs and an adhesion layer having portions over the LEDs. Electrically scattering particles adhere by electrostatic attraction to the adhesion layer portions, forming scattering regions that are self-aligned to the light emitting device. Between the LEDs the particle layer may be thinner.
It is an object of the invention to provide an improved LED module that overcomes the problems described above.